The present invention relates to an optical waveguide element for switching a path between a transverse electric (TE) polarized wave and a transverse magnetic (TM) polarized wave, and a receiving circuit including the optical waveguide element.
Along with the increase in information transmission amount, an optical wiring technology is gaining attention. In an optical wiring technology, information is transmitted between devices, boards, chips, or the like in an information processing device through optical signals by using an optical device using an optical fiber or an optical waveguide as a transmission medium. As a result, it is possible to improve a bandwidth limitation of an electrical wiring, which is a bottleneck in an information processing device which requests high-speed signal processing.
Optical devices have optical elements such as optical transmitters and optical receivers. These optical elements can be spatially coupled with each other using, for example, a lens after performing a complicated optical axis alignment for aligning the center position (light receiving position or light emitting position) of each optical element with the design position.
As a means for coupling each optical element, there is a technology of using an optical waveguide element instead of a lens. In the case of using the optical waveguide element, because light is confined within the optical waveguide and propagates, unlike the case of using the lens, complicated optical axis alignment is not necessary. Therefore, because an assembling process of an optical device is simplified, it is advantageous as a form suitable for mass production.
In the optical waveguide element, for example, silicon (Si) can be used as a waveguide material. In an optical waveguide element (Si waveguide) using Si as a material, an optical waveguide core, which substantially serves as a light transmission path, is formed using Si as a material. Then, surroundings of the optical waveguide core are covered by a cladding formed of, for example, silicon oxide (SiO2) or the like having a lower refractive index than Si. With such a configuration, because a difference in refractive index between the optical waveguide core and the cladding becomes extremely large, it is possible to strongly confine light in the optical waveguide core. As a result, it is possible to realize a small curved waveguide in which a bending radius is reduced to about several μm, for example. Therefore, it is possible to prepare an optical circuit having the size similar to an electronic circuit, and it is advantageous for reducing the entire size of the optical device.
In addition, when using the Si waveguide, mass production of an optical device in which elements having various functions are monolithically integrated on the same substrate is possible by applying a manufacturing process of a complementary metal oxide semiconductor (CMOS). Therefore, the optical device using the Si waveguide is advantageous for size reduction and cost reduction (refer to, for example, Non-patent Document 1 (IEEE Journal of selected topics quantum electronics, vol. 11, 2005 p. 232-240) and Non-patent Document 2 (IEEE Journal of selected topics quantum electronics, vol. 12, No. 6, November/December 2006 p. 1371-1379)).
On the other hand, in the Si waveguide, due to a large difference in relative refractive index between the core and the cladding, a difference in equivalent refractive index in a waveguide mode and a difference in group index tend to be large between a TE polarized wave and a TM polarized wave. Therefore, the Si waveguide has a polarization dependency.
Therefore, in a wavelength filter such as a Mach-Zehnder interferometer, a ring resonator, a grating, and an arrayed waveguide grating (AWG) constituted by a Si waveguide, a deviation of wavelength response characteristics between both polarized waves becomes large even at the same wavelength. Such a deviation of the characteristics becomes a cause of inter-channel crosstalk in a reception-side device or the like in a fiber transmission system, for example.
A light receiving element using the Si waveguide is formed by depositing a light absorption layer such as germanium (Ge) on the Si waveguide. A magnitude of a photocurrent converted by the light receiving element varies depending on polarized waves.
To eliminate the polarization dependency in the Si waveguide, there is a system in which a band-pass filter for a TE polarized wave and a band-pass filter for a TM polarized wave that extract the same wavelength are separately prepared (refer to, for example, Patent Document 1 (JP H6-201942A)). In this system, the band-pass filters for the TE polarized wave and the TM polarized wave are connected in parallel or in series. Then, the TE polarized wave and the TM polarized wave extracted from the band-pass filters are multiplexed by a polarization multiplexer. Therefore, the system of Patent Document 1 functions as a wavelength filter capable of extracting a specific wavelength without depending on polarized waves.
However, in the system of Patent Document 1, the TE polarized wave and the TM polarized wave are included in the light multiplexed by the polarization multiplexer. Therefore, even when, for example, a light receiving element is installed at a rear stage of the system, the above-described polarization dependency (in other words, variation in magnitudes of a photocurrent) remains in the light receiving element.
As another structure for eliminating the polarization dependency in the Si waveguide, there is a structure in which a polarization separating element and a polarization rotating element are provided at a front stage of a wavelength filter (refer to, for example, Patent Document 2 (JP 2009-244326A)).
In the structure of Patent Document 2, first, light is separated into a TE polarized wave and a TM polarized wave orthogonal to each other by the polarization separating element. Next, one of the polarized waves is rotated 90° by the polarization rotating element. As a result, light input to the wavelength filter is aligned to either the TE polarized wave or the TM polarized wave. Therefore, design of the wavelength filter is necessary only for either the TE polarized wave or the TM polarized wave, and the polarization dependency is eliminated. When the structure of Patent Document 2 is used, because light can be aligned to either the TE polarized wave or the TM polarized wave, the above-described polarization dependency in the light receiving element can be eliminated.
The polarization separating element can be constituted by, for example, using a directional coupler having two Si waveguides arranged next to each other. In the polarization separating element using the directional coupler, a core of the Si waveguide is formed to have a flat cross-sectional shape. This generates a difference in coupling action length of the directional coupler with respect to the TE polarized wave and the TM polarized wave. As a result, the TE polarized wave and the TM polarized wave can be output through different paths.
The polarization rotating element has a so-called eccentric double core structure in which two optical waveguide cores having different refractive indexes are overlapped. In this structure, an upper optical waveguide core having a smaller refractive index than a lower optical waveguide core is formed on the lower optical waveguide core. The upper optical waveguide core is covered with a cladding having a smaller refractive index than the upper optical waveguide core. As a result, an optical waveguide element in which a propagation center of light with respect to the lower optical waveguide core and a propagation center of light with respect to the upper optical waveguide core do not coincide with each other is constituted. In the polarization rotating element having the eccentric double core structure, an arbitrary rotation amount can be given to light propagating at a predetermined distance.